1. Field of the Invention
The present invention relates to an image forming apparatus which can form images on both sides of a paper sheet.
2. Description of Related Art
There is a type of image forming apparatus such as a copier, a printer, a facsimile, or a multifunction apparatus, which can form images by an electrophotographic method on both sides of a paper sheet (e.g., see JP-A-10-026857, JP-A-2005-315920 and JP-A-2007-217126).
This type of image forming apparatus includes a sheet storage portion for storing paper sheets, an image forming portion for forming images on the paper sheet fed from the sheet storage portion, a sheet ejector for delivering the paper sheet externally after the image forming portion forms images on one side or both sides of the paper sheet, a sheet-reversing unit for reversing sides of the paper sheet after the image forming portion forms images on one side of the paper sheet, a first conveying path from the sheet storage portion to the image forming portion, a second conveying path from the image forming portion to the sheet ejector, a third conveying path which branches from the second conveying path and reaches the sheet-reversing unit, and a fourth conveying path which extends from the sheet-reversing unit and joins the first conveying path.
When the image forming apparatus having the above-described structure forms image on both sides of the paper sheet, the paper sheet fed from the sheet storage portion passes through the first conveying path and is supplied to the image forming portion, so that the image forming portion forms images on one side (first side). This paper sheet with images formed on one side passes through the second conveying path and the third conveying path so as to be led to the sheet-reversing unit, which reverses sides of the paper sheet. After that, the paper sheet passes through the fourth conveying path and is led back to the first conveying path. This paper sheet that is led back to the first conveying path after the sides thereof are reversed is supplied again to the image forming portion, so that images are formed on the opposite side (second side opposite to the side on which the images are formed in the first time). The paper sheet with images formed on both sides is delivered externally from the second conveying path by the sheet ejector.
This type of image forming apparatus has a structure for opening a conveyance guide to release the fourth conveying path so that a jammed paper sheet can easily be removed if jamming of the paper sheet occurs while the paper sheet with images formed on one side passes through the fourth conveying path to be led back to the first conveying path (e.g., see JP-A-2005-315920).
In particular, a high speed apparatus conveys two or more paper sheets continuously in the fourth conveying path that is a two-sided conveyance path, and it adopts a structure of opening a large conveyance guide to release the fourth conveying path so that jammed paper sheets can be removed at one time if jamming of two or more paper sheets occurs in the fourth conveying path. In this structure, it is desirable that the jammed paper sheet in the two-sided conveyance path can be removed easily. Further, it is desirable if the structure of the conveyance guide can be simplified for cost reduction.
In addition, when the two-sided printing of a plurality of sheets are performed, image forming on the paper sheet supplied from the sheet storage portion and printing on the back side (second side) of the paper sheet conveyed from the two-sided conveyance path are performed alternately from a midpoint of the printing job. The paper sheet with images printed on one side is temporarily in a standby state in the fourth conveying path. In this case, if the paper sheet is in the standby state in a curved portion of the conveying path or the like, the paper sheet may be curled. Therefore, it is necessary to adopt a structure in which the paper sheet can be in the standby state without being curled in the fourth conveying path.